In order to form a basis for understanding the detailed mechanism of oxygen transport in biological systems, we wish to determine, by single crystal X-ray diffraction techniques, the structures of several oxygen-carrying chelates of cobalt and perhaps of other transition metals. It is presently feasible to determine the geometry and interatomic distances at the site of oxygen binding with a high degree of accuracy; this is so, even though most of the work in this field to date has been of fairly low precision (thus its accuracy is hard to assess). We have improved this precision by an order of magnitude and propose to extend our work to the synthesis and structural determination of various substituted bis(salicylaldehyde)-1,1,2,2-tetramethylethylenediimine cobalt(II) complexes in their oxygenated forms. Variation of the substituents on the salicylaldehyde ring and of the axial base affect both the oxygen-carrying properties of the complexes and their ease of crystallization; in our work we will try to explain the causes of these variations in behavior. Until a technique is available for determining directly the metal-oxygen geometry in metalloproteins -- and none is available now -- work such as this on model compounds continues to be important.